What is a prime number?
A prime number is a whole number greater than 1 that can be divided evenly only by 1 and itself. The first primes are 2, 3, 5, 7, 11 and 13.
A number with more than two factors, like 6, which can be divided by 1, 2, 3 and 6, is called a composite number. Every whole number greater than 1 is either prime or can be written as a product of primes in exactly one way. For example, 12 = 2 × 2 × 3.
The Greek mathematician Euclid proved more than 2,000 years ago that there are infinitely many primes. They become less common as numbers get larger, but they never run out.
Why primes matter
Primes are used to protect information online. Encryption systems rely on the fact that multiplying two very large primes is easy, but finding the original primes from the result is extremely difficult. The largest known primes have tens of millions of digits.
Did You Know?
Twin primes are pairs of primes that differ by 2, such as 11 and 13 or 17 and 19. Nobody has yet proved whether there are infinitely many of them.